This invention relates to the curing of films of polyamic acid or salts thereof to form polyimides. More specifically, the invention relates to the use of ultraviolet radiation to cure films of polyamic acid or salts thereof.
Polyimides are typically prepared by a condensation polymerization reaction between organic diamines and tetracarboxylic acid dianhydrides, and demonstrate excellent high temperature and dielectric properties. Because of the properties of polyimides, they have found broad utility in many areas.
Present commercial practice is to purchase polyimide films themselves or polyamic acid, the precursor of polyimides, in a liquid, and thus castable, form. The polyamic acid, once cast and the solvent removed, can be cured or imidized at elevated temperatures to the polyimide form. This curing temperature is in the range of 650.degree. F.
The problem with this practice is, of course, that the number of substrates available for application of a curable polyamic acid or salt thereof thereto becomes somewhat limited, i.e. they must be in essence temperature-resistant. Yet, in many instances, it would be desirable to use substrates in conjunction with a polyimide which would not be heat stable at the normal elevated curing temperatures for formation of the polyimide. Such applicability has not heretofore been available.
It has now been ascertained that ultraviolet radiation can contribute to the curing of films of polyamic acid or salts thereof to a polyimide. The condensation polymerization reaction involved in the curing process can therefore be undertaken at a much lower temperature than was normally thought necessary. This allows use of substrates, in conjunction with polyimides, which are not normally stable to excessive temperatures.
Furthermore, by use of the ultraviolet curing, films of polyamic acids or their salts attain the status of being photodefinable, i.e. a mask can be selectively utilized to cure selected areas of the polyamic acid or salt film in an imagewise fashion, thereby allowing removal of the unexposed portions, again in an imagewise fashion.